1. Field of the Invention
The present invention relates to a bending machine capable of bending a plate-like workpiece while protecting it from damage.
2. Related Art
In a case where a plate-like workpiece is bent, so-called "spring back" takes place, which is a phenomenon causing the bent workpiece to be slightly deformed from the desired shape due to an elastic restoration of the bent workpiece.
Hitherto, a workpiece has been bent by a surplus amount to take the aforesaid "spring back" into account so as to obtain the desired shape. In order to bend the workpiece as described above, it has been known, for example, Japanese Published Unexamined Utility Model Application No. (U) 61-152325, to adapt a bending machine in such a manner that a bending blade is disposed at the leading portion of a rocker arm to be rotated in a vertical direction by a double action actuator so as to bend the workpiece by the bending blade which is moved by the vertical movement of the rocker arm, and an eccentric member for supporting the base portion of the rocker arm is rotated so as to horizontally displace the overall body of the rocker arm, so that the amount of the "spring back" is corrected.
However, a problem arises in that the quality of the product deteriorates in a case where a workpiece, the surface of which has been treated with a surface treatment such as a coating or zinc plating, is bent because the aforesaid bending machine performs the bending operation in a state where the bending blade rubs the workpiece, and therefore the surface of the workpiece is damaged. What is worse, the cost of a control system cannot be reduced because complicated control must be performed to obtain a desired bending angle.
In a case where a panel arranged as shown in FIG. 19 is formed from a plate-like workpiece a, a shorter side edge b of the workpiece a is bent first, then the workpiece a is rotated by 90.degree. so that a clamping die or an upper die is adjusted to the width to be bent which is aligned to the inside of the bent portion of the shorter side edge b, and then a longer side c is bent.
In a bending operation of the aforesaid type, the upper die for bending the longer side edge c must have a length obtained by subtracting thickness corresponding to those of two workpieces a from the length of the longer side because the bent shorter side edge b has been raised. In addition, the die must be separated after it has bent the longer side edge c in such a manner that the die cannot be caught by the shorter side edge b. Technology capable of achieving the aforesaid requirement has been disclosed in Japanese Patent Publication No. 1-16568 in which the interference is prevented by arranging the structure in such a manner that an opening die, which can be rotated by a pin or a link, is disposed on a corner of the upper die.
However, each of the aforesaid opening dies is structured in such a manner that it is rotated by the dead weight thereof to prevent the interference. Therefore, when the ram starts moving upwardly at the time of separating the die, the opening die moves to prevent the interference while being in contact with the upper surface of the workpiece which has been molded. Also at the time of the bending operation, the opening die, which has been moved to a position at which the interference can be prevented with the downward movement of the ram, moves to the position at which molding is performed while being in contact with the upper surface of the workpiece. Therefore, a problem arises in that the quality of the product deteriorates excessively due to damage caused from rubbing in a case where the workpiece is made of colored iron plate or which has been coated with paint.